the neutron star stage. They just disappear
into their dark prisons. Also, a neutron star
in a binary system might become a medium
size black hole if enough mass from its com
panion accumulates on it to push it over a
critical edge.
In addition, a pulsar has been discovered
orbiting another compact object, separated
by a distance comparable to the sun's diame
ter. "That's like two soccer balls ten miles
apart," said Joseph Taylor of Princeton Uni
versity, "exerting a profound influence on
each other." Gravity is irresistibly drawing
the two stars closer. "When they meet in 300
million years," says Taylor, "they will merge
into a black hole."
The monster black holes that may be at
the centers of galaxies probably formed as
the galaxies were getting organized. Whirl
pools of matter would have gravitated
inward to the collapsing core. Michael
Penston of Britain's Royal Greenwich Ob
servatory believes his team has actually
weighed a galactic black hole by using IUE
to measure the speed of gas orbiting it. "The
black hole in NGC 4151," he says, "has the
mass of 100 million suns." The diameter of
such a black hole would stretch three
fourths of the way from earth to Jupiter.
44 ALWAYS WANTED to know what a
black hole would look like," said Uni
versity of British Columbia physicist
William Unruh. He was about to show
me a computer movie he made to fulfill that
fantasy. "You can't see a black hole. Just its
effects. I imagined myself in a spaceship in
orbit around a 10,000-solar-mass black
hole, looking at the constellation Orion."
The movie began from a point in that or
bit with an unobscured view of Orion. Soon
something that looked like a dark amoeba
with a twirling crown of stars moved across
the screen. The stars were not real. Rather,
the gravity near the black hole's horizon had
made a profound mirage, bending the light
from every star in the sky like a lens and
focusing it into second images of those stars,
creating a halo around the hole.
As the spacecraft went behind the black
hole, Orion turned into a pinwheel. I could
watch the three stars in the hunter's belt
being pulled apart and hurled into the black
hole's crown of stars. As the spacecraft
came out from behind the amoeba, Orion
quickly reassembled.
Even though this star twirling was an illu
sion, Unruh's movie was compelling testi
mony to the gravitational strength of black
holes. Black holes literally can tear stars
apart. Stars could well be what feed the
black holes in active galaxies. Like New
ton's apple in an accretion disk, pieces of
stars would liberate enormous energies and
create colossal fireworks on their swirling
descent into extinction.
"Fifteen years ago black holes were on the
fringe of respectability,"
said Anthony
Readhead. "Now we must take them seri
ously. No other mechanism can explain the
energies released by active galaxies. Nucle
ar explosions won't do. Throwing stars into
black holes is perhaps a 50 times more effi
cient way to generate energy."
Quasars may simply have been a stage
most galaxies went through as their black
holes formed and swept up stars and gas
within their gravitational reach. In those
early days the universe had expanded only
to a fourth its current size. Galaxies were
close enough to collide frequently, cannibal
izing each other. So the quasar epoch may
also reflect the prevalence of food back then.
The age of quasars has ended. "Apparent
ly, most galaxies have used up the fuel near
their cores," said Roger Blandford of Cal
tech. "But they are left with their central
black holes, quietly waiting for more food."
Occasionally there may still be acts of can
nibalism. A strange dark band streaks the
agitated center of the nearby galaxy Centau
rus A.
"That band," said University of
Washington's Bruce Balick, "suggests that
Cen A has ripped apart a galaxy perhaps as
large as the Milky Way. Some of its rem
nants now appear to be streaming in to feed
Cen A's monster."
Does the Milky Way have a slumbering
monster of its own?
"Radio astronomers have found a curious
energetic dot in our galactic center," said
Balick. "It could well be a black hole."
We might even have been active ourselves
recently. "There's an expanding ring of gas
clouds around our galactic center,"
said
Dennis Downes at the French Institut de
Radio Astronomie Millimetrique. "What's
causing it? Was there a big explosion?"
The Once and Future Universe
735